1. Field of the Invention
The present invention relates to a solid-state imaging device having a charge-coupled device (hereinafter also referred to as “CCD”).
2. Description of the Related Art
With solid-state imaging devices used in consumer and professional video cameras, as well as in the digital still cameras that have undergone such amazing growth in recent years, the size of the charges handled by CCD pixel components (hereinafter also referred to simply as “pixel components”) that are a result of reduced cell size has increased tremendously as cameras have become smaller and include more pixels. On the other hand, horizontal transfer CCD components that send signal charges to an amplifier (hereinafter also referred to simply as “horizontal CCD components”; in the following description the word “transfer” may be omitted as appropriate) need to have lower voltage requirements so that they can be driven by battery, and a major increase in the amount of charge handled has yet to be achieved. Consequently, this creates a situation in which an image cannot be outputted during the charge transfer of a signal produced by a large quantity of light (hereinafter also referred to as “large optical volume signal”), such as when capturing an image of a subject with high brightness, in indoor and outdoor image capture. This phenomenon will be described with reference to FIG. 9. FIG. 9 is a schematic of a common solid-state imaging device. When capturing an image of a subject with high brightness, the large amount of light generates a large quantity of electrons at the photodiodes 91 of a pixel component 93, and these overflow into a vertical CCD 92. The vertical CCD 92 is constantly performing transfer to a horizontal CCD 95, so that the horizontal CCD 95 fills up with electrons in the region 33 corresponding to the pixel component. These electrons go over the level that can be handled as a signal and thus are called unwanted electrons. At this point, unless the unwanted electrons can be eliminated quickly to an unwanted electron eliminator 96 that has been created in contact with the horizontal CCD 95, the signal charge (that is, the electrons) will completely bury the horizontal CCD 95 in the region 34 corresponding to an optical black component (hereinafter referred to as “OB component”) 94, making it impossible to detect the 0 level serving as a reference signal in the camera, and preventing the output of an image.
An unwanted electron eliminator in a conventional solid-state imaging device has been disclosed in JP H9-223788A, for example. FIG. 10 is a schematic cross section of the conventional horizontal CCD component including an unwanted electron eliminator disclosed in JP H9-223788A. In FIG. 10, reference numeral 101 is an n-type semiconductor substrate, 102 is a p-type well, 103 is an n-type region that is a horizontal CCD component, 104 is an n-type region that is a buried channel of a potential barrier, 105 is an n-type region that is an unwanted electron eliminator, 106 is a p-type region that separates devices, 107 is polycrystalline silicon that serves as a first horizontal charge transfer electrode, 108 is a gate insulating film, 131 is a thick oxide film, and 130 is an insulating film. The unwanted electron eliminator and the buried channel of the horizontal charge transfer component are formed with the same impurity profile. The broken line 109 indicates the horizontal direction on the substrate surface within the regions 104 to 106.
With a conventional configuration, however, if the n-type region 103 that is a horizontal CCD component, the n-type region 104 that is a buried channel of a potential barrier, and the n-type region 105 that is an unwanted electron eliminator are merely formed with the same impurity profile, there will be a decrease in the ability to sweep away the unwanted electrons that flow in from the pixel component, making it impossible to output an image, or decreasing the amount of charge that can be handled by the horizontal CCD. This problem will be described through reference to FIGS. 11A to 11C. FIGS. 11A to 11C are schematic diagrams of the impurity distribution and potential distribution in the region along the broken line 109, going through the n-type region 103, n-type region 104, and n-type region 105 in FIG. 10. The following problems will be encountered when the impurity concentration of the n-type region 104 (hereinafter “impurity concentration” also will be referred to simply as “concentration”) is relatively low. The impurity distribution 111 in FIG. 11A results when the n-type region 104 has a relatively low concentration. The potential 115 in FIG. 11B is the potential in a state in which power source voltage applied from around the solid-state imaging device has depleted the n-type region 103 and the n-type region 104, but the n-type region 105 remains undepleted. The potential drops from the potential 115 to the potential 113 when unwanted electrons 114 flow in from the pixel component in the case of a large optical volume signal. However, since the electron elimination distance 116 within the n-type region 104 is long, the unwanted electrons 114 cannot be eliminated, and the horizontal CCD corresponding to the pixel component overflows with unwanted electrons 114. Accordingly, even the horizontal CCD of the OB component fills up with the unwanted electrons 114, and consequently no image can be outputted when there is a large optical volume signal.
Meanwhile, the following problems will be encountered when the impurity concentration of the n-type region 104 is relatively high. The impurity distribution 112 in FIG. 11A results when the n-type region 104 has a relatively high concentration. The potential 117 in FIG. 11C is the potential in a state in which power source voltage applied from around the solid-state imaging device has depleted the n-type region 103 and the n-type region 104, but the n-type region 105 remains undepleted. The potential drops from the potential 117 to the potential 119 when unwanted electrons 118 flow in from the pixel component in the case of a large optical volume signal. In this case, the electron elimination distance 120 within the n-type region 104 for eliminating the unwanted electrons 118 is shorter than the above-mentioned electron elimination distance 116, and therefore poses no problem in terms of eliminating unwanted electrons. However, the potential at which signal charge can be accumulated with the horizontal CCD is only the potential 119, which means that the amount of charge that can be handled by the horizontal CCD decreases.